When a provider error (401/429/rate-limit) causes the agent to return
without producing a new assistant reply, the WebUI should emit an
apperror event so the user sees an inline error. However, the detection
logic scanned ALL messages in result['messages'] — which includes the
full conversation history. If any prior turn had an assistant response,
_assistant_added would be True and the apperror would be silently
skipped, leaving the user staring at a blank response.
Extract a helper _has_new_assistant_reply(all_messages, prev_count)
that only inspects messages beyond the pre-turn history offset. Apply
it to both the main detection path and the self-heal/retry path.
Tests: 15 new cases covering history masking, empty content, whitespace,
edge-case shrinks, and multi-assistant scenarios.
